1. Field of the Invention
The present invention relates to a centrifugal compressor including a shroud covering the edges of the vanes of a compressor wheel mounted on a rotary shaft with a predetermined clearance left therebetween, and also relates to a centrifugal turbine including a shroud covering the edges of the vanes of a turbine wheel mounted on a rotary shaft with a predetermined clearance left therebetween.
2. Description of the Related Art
In a centrifugal compressor adapted to compress air axially drawn by the compressor wheel mounted on a rotary shaft and to discharge the compressed air radially outwards, an enhancement in compression performance can be provided by keeping the clearance defined between the edges of the vanes of the compressor wheel and an inner surface of the shroud to a small size. However, there is a limit to decreasing the clearance due to the limitation of processing accuracy and the thermal expansion of the shroud, caused by the heat of compression of the air. Therefore, a centrifugal compressor has been proposed in Japanese Patent Application Laid-open No. 5-196598, in which a step is formed on an inner wall surface of a shroud opposed to the downstream end of a compressor wheel to decrease the sectional area of a flow path, so that the air leaking into the clearance, is dammed up by the step to prevent a reduction in compression efficiency.
However, the above prior art centrifugal compressor suffers from a problem that the air leaks through spaces between the vanes of the compressor wheel into the clearance and for this reason, the vanes cannot provide a sufficient centrifugal force to the leaked air and as a result, a reduction in compression efficiency is unavoidable. The problem that the clearance between the edges of the vanes and the inner surface of the shroud is varied due to the thermal expansion, as described above, also arises in a centrifugal turbine.
The present invention has been developed with the above circumstances in view, and it is an object of the present invention to suppress the variation in clearance defined between the edges of the vanes of the compressor wheel or the turbine wheel and the inner surface of the shroud to prevent a reduction in performance.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a centrifugal compressor including a shroud covering edges of vanes of a compressor wheel mounted on a rotary shaft with a predetermined clearance therebetween. A vertical section of the shroud includes an upstream portion extending in an axial direction of the rotary shaft, and a downstream portion curved radially outwards of the rotary shaft and extending from the downstream end of the upstream portion. The thickness of the downstream portion is increased from the upstream side toward the downstream side.
With the above arrangement, the thickness of the downstream portion curved radially outwards and extending from the upstream portion of the shroud is increased from the upstream side toward the downstream side. Therefore, the rigidity of the downstream portion can be increased to suppress the axial displacement due to the thermal expansion. Thus, it is possible to prevent the variation in the clearance defined between the compressor wheel and the downstream portion of the shroud which exerts a large influence in the compression performance of the centrifugal compressor, and to suppress the thermal expansion of the shroud thereby minimizing the reduction in performance.
According to a second aspect and feature of the present invention, there is provided a centrifugal turbine including a shroud covering the edges of the vanes of a turbine wheel mounted on a rotary shaft, with a predetermined clearance left therebetween, a vertical section of the shroud including a downstream portion extending in an axial direction of the rotary shaft, and an upstream portion curved radially outwards of the rotary shaft and extending from an upstream end of the downstream portion. The thickness of the upstream portion is increased from the downstream side toward the upstream side.
With the above arrangement, the thickness of the upstream portion curved radially outwards and extending from the downstream portion of the shroud, is increased from the downstream side toward the upstream side. Therefore, the rigidity of the upstream portion can be increased to suppress the axial displacement due to thermal expansion. Thus, it is possible to prevent the variation of the clearance defined between the turbine wheel and the upstream portion of the shroud which exerts a large influence on the power performance of the centrifugal turbine, and to suppress the thermal expansion of the shroud, thereby minimizing the reduction in performance.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.